Flue gas desulfurization processes, also referred to as FGD, can conveniently be categorized by the manner in which the sulfur compounds removed from the flue gases are eventually produced for disposal.
One category is termed "throwaway process" in which the eventual sulfur product is disposed of as waste. Disposal can include a landfill or pond. The processes in this category involve wet scrubbing of the flue gases for absorption, followed by various methods for neutralizing the acidity, separating the sulfur compounds from the scrubbing liquor, and usually recycling at least part of the scrubbing liquor.
A second category is the gypsum processes, which are designed to produce gypsum of sufficient quality either for use as an alternative to natural gypsum or as a well-defined waste product. As with the throwaway processes, this category involves wet scrubbing for absorption followed by various methods of neutralizing the lime or limestone and recovering the sulfur compound. The sulfur dioxide (SO.sub.2) is first absorbed in the solution and then the dissolved sulfur dioxide reacts with lime (Ca(OH).sub.2) or limestone (CaCO.sub.3) to produce calcium sulfite (CaSO.sub.3). The sulfite species in the slurry are oxidized by oxygen present in the flue gas producing hydrated calcium sulfate (CaSO.sub.4 2H.sub.2 O), herein referred to as gypsum. Since gypsum is more desirable than calcium sulfite, additional oxidation is carried out to completely convert calcium sulfite to gypsum, which is an additional oxidation step, also called forced oxidation process.
The major drawbacks of the wet flue gas desulfurization process are scaling and plugging in the absorber; reheating is required for stack gas buoyancy; difficulty is experienced in growing gypsum crystals for efficient solid/liquid separation; and there is minimal removal of nitrogen oxides (NO.sub.x).
Dry flue gas desulfurization involves sorption of sulfur dioxide in solids, such as lime, metal oxides, and activated carbon. Dry sorption processes take place at higher temperatures than wet processes. Generally, regeneration processes for carbon adsorbents occur at elevated temperatures above 120.degree. C., and particularly, above about 400.degree. C. As the temperature increases during the regeneration process, the acid reacts with the carbon and forms carbon dioxide. In addition, the carbon will reduce SO.sub.3 to SO.sub.2. This reaction produces carbon monoxide and carbon dioxide. These reactions cause carbon loss that consequently affect the process economics and effectiveness of the carbon adsorbent.
There is a need for a system that uses low temperatures to regenerate carbon adsorbents while reducing carbon loss that produces gypsum from the regeneration by-products. There is also a need for a dry low temperature process to regenerate carbon used in flue gas desulfurization that produces gypsum while eliminating a forced oxidation step. Also, there is a need for a cost effective system to produce gypsum from a dry low temperature flue gas desulfurization process that produces environmentally safe products and provides easy operation.